1. Field of the Invention
The present invention relates to a developer for electrophotography and a method for electrophotographic developing using the same, which is useful for developing an electrostatic latent image formed by an electrophotographic method, an electrostatic recording method or an electrostatic printing method.
2. Prior Art
In general, according to an electrophotographic method, a copy image is prepared by developing an electrostatic latent image formed on a photo-conductor using a developer to form a toner image, and optionally transferring the toner image on a transfer member, followed by fixing the toner image to the transfer member. In the electrophotographic method described above, heretofore, a two-component developer has been known which consists essentially of a non-magnetic toner and a magnetic carrier.
A method for electrophotographic developing using the two-component developer (hereafter referred to as a "two-component method for developing") comprises the steps of: (1) mixing a non-magnetic toner and a magnetic carrier in a predetermined ratio; (2) subjecting the mixture to a triboelectric charging treatment until the non-magnetic toner is charged with a desired polarity; and (3) fixing only the non-magnetic toner (and not the magnetic carrier) on a photo-conductor. Although the two component method for developing has an advantage in that the image transfer to a paper sheet is facile due to a high electric resistance of the non-magnetic toner, the method has the following drawbacks:
(1) A mixer for mixing the non-magnetic toner and the magnetic carrier must be handled with care in order to afford an adequate triboelectric charging to the two components; PA1 (2) Since only the non-magnetic toner is consumed (i.e. fixed) during the developing step, there is a need for a toner density sensor to control the toner-to-carrier ratio, resulting in a large and complex developing device; and PA1 (3) During the use of the two-component developer, the magnetic carrier is exhausted so that the life of the two-component developer is terminated within a relatively short period of time. PA1 (1) preparing a developer for electrophotography comprising a magnetic carrier, and a magnetic toner which includes a magnetic powder having at most 100 oersted of a coercive force (Hc) and satisfying the following formula: EQU Ms/Mr.gtoreq.20 PA1 wherein Ms represents a saturation magnetization and Mr represents a residual magnetization; PA1 (2) preparing a developing device equipped with a magnetic roll, and a sleeve encompassing the magnetic roll; PA1 (3) preparing a photo-conductor on which an electrostatic latent image is formed; PA1 (4) subjecting a surface of the sleeve to a magnetic attraction using the developer for electrophotography; PA1 (5) subsequently rotating the magnetic roll at a peripheral speed of 300 mm/sec or faster and the sleeve at a peripheral speed of 50 mm/sec or faster in opposite directions, to carry the developer for electrophotography; and PA1 (6) subsequently contacting the developer with the photo-conductor on which the electrostatic latent image is formed to develop the electrostatic latent image.
In contrast to the two-component developer, a single-component developer which includes a magnetic powder and a binder resin as the main ingredients and which is freed from a carrier, has been proposed. Due to the absence of a carrier in a single-component method for developing, it is not necessary to control the toner density. Therefore, the toner density sensor, described above, is not necessary. However, in order to secure excellent accuracy of the space between a blade and a sleeve surface of a developing device and the space between the sleeve surface and a photo-conductor, it is necessary to have a method for development that forms a uniform magnetic brush of toner on the surface of the sleeve.
In order to overcome the disadvantages described above in the two-component developer and the single-component developer, various developers which include magnetic toners and magnetic carriers have been proposed.
For example in U.S. Pat. No. 4,640,880, the developer includes both a magnetic toner and a magnetic carrier. In comparison with the conventional two-component developer, this developer is distinguished by the usage of a larger (20% or more) amount of magnetic toner. In other words, the ratio of the magnetic carrier to the magnetic toner in the developer is small, making it difficult for the relatively small amount of magnetic carrier to contact with the magnetic toner. For this reason, the desired triboelectric charging of the magnetic toner cannot be obtained. In order to avoid this problem, the developing device must have a high-speed rotating sleeve and a high-speed rotating magnetic roll so that a sufficient triboelectric charging to the magnetic tone can be attained. Therefore, the capacity of the main motor must be expanded, or an additional high-speed motor for driving the sleeve and the magnetic roll is needed, resulting in a a large-sized and/or costly developing device.